Synthetic aperture radar, or SAR, has become an invaluable remote-sensing tool. To achieve the finest resolution, focused SAR is used, wherein some process is used to modify received signal histories to mitigate the detrimental effects of scatterer motion through range and doppler resolution cells. One approach to focussed SAR involves the use of polar format (PF) processing.sup.1. An alternative approach is based upon the range migration algorithm, or RMA, also known as the .OMEGA.-K algorithm and wave number processing. Both the PF and RMA techniques have their advantages and disadvantages.sup.2. PF is typically less computationally intensive and more memory efficient, but systems based on the RMA are capable of producing well-focused, fine-resolution images over a wider area scene. FNT .sup.1 Walker, J. L., "Range-Doppler Imaging of Rotating Objects," IEEE Transactions on Aerospace and Electronic Systems, Vol. AES-16, No. 1, January 1980, pp. 23-52. FNT .sup.2 Cafforio, C., C. Prati, and F. Rocca, "SAR Data Focusing Using Seismic Migration Techniques," IEEE Transactions on Aerospace and Electronic Systems, Vol. 27, No. 2, March 1991, pp. 194-206.
More specific advantages of the RMA over conventional PF processing include the following:
1. The RMA provides an exact solution to the problem of range curvature. Fine-resolution, large scene images produced using the RMA do not suffer the geometric (keystone) distortion or residual quadratic phase errors inherent in conventional PF processing; PA1 2. The RMA can produce large scene size images free of subpatch and subswath boundaries. Such images are better suited for coherent-based image enhancement and exploitation techniques; and PA1 3. The RMA is directly capable of processing spotlight, stripmap, and scan-mode SAR images. These advantages become increasingly significant with improvements in SAR system resolution and area coverage. The RMA is particularly useful in fine-resolution SAR systems operating at low frequencies, typically in systems designed for foliage penetration (FOPEN) and ground penetration (GPEN) applications.